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As demand for renewable energy sources increases, so does the need for modern dismantling technologies for underwater use. For example, to bring offshore wind turbines up to more power, old steel frames must first be dismantled below sea level so that they can later be rebuilt larger. The Fraunhofer Institute for Material and Beam Technology IWS in Dresden has now found a technological approach to use lasers as particularly efficient, environmentally friendly and energy-saving cutting tools in water.

Adaptation to conditions under water

Cutting metal with lasers is, of course, not a fundamentally new approach, but it is usually done in a dry environment. Infrared or other rather long-wave laser beams are used. The resulting molten metal is removed coaxially with the beam by auxiliary gases. In the sea, however, conditions are quite different: Water scatters long-wavelength light in all directions, causing a large part of the laser power to dissipate after only a short distance. Complex piping systems would also be needed for the auxiliary gas.

Ready for use with industrial partners

To cut steel and other metals under water, the Fraunhofer researchers are therefore relying on particularly short-wavelength green lasers, whose cutting capability is also given in water. At the same time, the researchers are using the water itself as a tool to expel the resulting melt from the kerf with pressure. This eliminates power losses, extra gas lines and other disadvantages. This process, which has so far only been tested in the laboratory, is now to be brought to operational maturity in the next step together with industrial partners.

Kilowatt-class green lasers as the key to underwater cutting

The use of green lasers with much shorter wavelengths than most industrial lasers in use today has only been made possible by the fact that green lasers in the class with more than one kilowatt of power are now available. Only these lasers achieve the necessary cutting power for the tasks envisaged under water. In the future, even shorter-wavelength versions with blue lasers are conceivable. Such short-wave lasers penetrate water without great losses and would thus be perfectly suited.

Water as cutting medium

Water as a substitute for the cutting gas required in a dry environment also offers a significant advantage: Unlike gases and gas mixtures, water resists compression, which means that as a cutting medium it can remove the melt residue at the interface with less effort and loss of time. In addition, among other things, the gas lines that were previously necessary can be eliminated.

Less energy loss - but more power, flexibility and safety

According to the Dresden engineers, underwater laser cutting has a number of advantages over the cutting methods commonly used today with saws, wire cutters and plasma cutters: "The process requires comparatively little energy and the power transmission is more efficient," emphasizes project manager Dr. Patrick Herwig, who heads the laser cutting group at the Fraunhofer IWS. This approach also allows the construction of particularly compact underwater robots with laser attachments. Because these could be smaller and more efficient than today's automatic saws, it would be easier to reach areas of underwater structures that were previously difficult to access. Unlike sawing, for example, dismantling teams would not have to continuously load future cutting lasers with new blades or other consumables. In addition, such a system would not generate waste or release hazardous materials into the atmosphere. This advantage would be particularly important in the demolition of old nuclear power plants. There, too, steel components often have to be dismantled under water first. If cutting gas were used here, the bubbles could carry radioactive waste to the water surface. This problem is also eliminated with laser underwater cutting.

Water: Friend instead of foe

Professor Christoph Leyens, director of the Fraunhofer IWS, describes the economic and social relevance of the innovative technology: "70 percent of the earth consists of water.In the future, mankind must increasingly use these offshore reserves to develop and expand environmentally friendly energy sources.For this we need new underwater manufacturing technologies like our laser cutting solutions.Until now, water has been seen as the 'enemy'.We are reversing that and understanding it as a 'friend'."In the next step, the researchers want to further develop their lab-scale proven concept into practically applicable systems.Fraunhofer IWS contributes its expertise in laser technology, system and analysis technology and for the design of complete systems.The scientists are currently looking for industrial partners to outline concrete application scenarios, experiences and challenges as well as to accompany the concrete technology development.